Inferring a user's activity may be based on the knowledge of the room in which the user is located. For example, knowing whether the user is in the living room, in the kitchen or in his bedroom might be indicative of the user's activity. It may be desirable to use an RFID tag system for informing a remote recipient, for example, in which room of a building the user is presently located.
The use of an RFID reader is described in, for example, a published patent application No. WO 2005/071597. There, an RFID tag array-based “smart floor” system for navigation and location determination for guiding individuals includes a plurality of spaced apart RFID tags. Each RFID tag has memory having information stored therein including positional information and attributes of objects or structures disposed in proximity to the tags. The tags convey radio frequency (RF) signals including the positional information and the attributes in response to received electromagnetic excitation fields. The RFID tags are embedded within or rigidly attached to a walking surface, such as carpet pad, tile and wood flooring.
Long range RFID tag systems operating in the ultra high frequency (UHF) band have a range that is typically 12 m in line of sight (LOS) conditions. This range could be drastically reduced by any blockage of the RFID tag or RFID reader caused by various kinds of obstacles such as people or furniture that results in a shadowing effect.
In a preferred embodiment of an RFID system, the RFID reader communicates with passive RFID tags placed in judiciously chosen regions in, for example, different rooms. Each of the RFID tags includes a directive antenna that enables the RFID tags placed in a given room to have a range that reaches relevant locations within the given room. Advantageously, the directive antenna prevents or reduces the radiation range of the RFID tag from extending to an adjacent room sharing with the room where the RFID tag is attached to the same wall, ceiling or floor. The RFID tags are distributed in respective regions in a given room that are separated from one another in a manner to mitigate the shadowing effect. Advantageously, the RFID tags receive the interrogating RF signal transmitted only from a transmitter of the RFID reader situated, typically, in the room where the RFID tags are located. Conversely, advantageously, the RFID reader receives the RF signal transmitted, typically, only from RFID tags situated in the room where the RFID tag is located.
Because of multipath frequency selective fading, encountered in indoor environments, significant level variations of the received RF signal are experienced even within a distance of a few centimeters. FIG. 5 shows a graph presenting an example of variations of magnitudes of a received RF signal at UHF frequency within an indoor area having coordinates X and Y of 2 m×2 m, respectively. As shown in FIG. 5, variation of the RF signal of up to 40 dBm could be noticed over distances of a few tens of centimeters. Such fast fading signal could, disadvantageously, prevent the activation of one RFID tag even for a transmission from a close RFID reader, situated in the same room; whereas, another RFID tag, located in a nearby room could, disadvantageously, be activated and thus read by the RFID reader located in the room in which the one RFID tag is located. As illustrated in the graph of FIG. 6, a situation may happen in which an RFID tag located in a so-called fade at a region 66 with respect to a signal transmitted by an RFID reader situated in the same room. On the other hand, an RFID tag situated a different room might be located on a so-called crest 68.
In a preferred embodiment of an RFID system, the RFID tags are arranged in sets such as in pairs. In such an arrangement, the RFID tags of a given tag pair are close enough to each other to be considered as being located substantially in the same region of the room, yet far enough in a manner to mitigate the aforementioned well known small scale fading due to multipath propagation.
In a preferred embodiment of an RFID system, a long range RFID reader included, for example, in a smartphone or tablet may, advantageously, share common circuitry that also performs the functionality of a smart phone. For example, an inertial or motion sensor that is used for smart phone purposes in a smartphone can also be used for initiating the communication with the RFID tags when a movement of a user is detected. This feature, advantageously, reduces the energy consumption of the RFID reader for preserving a charge in a battery of the mobile device. Advantageously, the smartphone can, selectively, send the user location information via the cellular network if the user enables this feature or can, selectively, prevent sending the user location information when the user disables this feature.